Does not fail on startup when one service is not available even when using `serviceHealthCheck`

I think this bug was already reported in the previous repo: https://github.com/apollographql/apollo-server/issues/3243
and is also loosely related to this other issue: https://github.com/apollographql/federation/issues/235.

According to the [documentation](https://www.apollographql.com/docs/federation/api/apollo-gateway/#servicehealthcheck) when using the `serviceHealthCheck` option it should fail on startup if a service doesn't return a successful healthcheck.
But it does not... an error about service definitions is logged and the gateway stays in a broken state with no valid schema.

# How to reproduce?
Simply by creating a gateway server with a service that simply does not exist in the list.
Note that activating `serviceHealthCheck` does not make any difference due to the problem highlighted bellow.

```javascript
const gateway = new ApolloGateway({
serviceList: [{name: "potato", url: "http://potato"}],
serviceHealthCheck: true,
});

const server = new ApolloServer({
gateway,
subscriptions: false,
});

server.applyMiddleware({ app, path: '/graphql' });
```

# First issue: shouldn't health checks be run before anything?
I think there is a problem here:
https://github.com/apollographql/federation/blob/a99f9350ffd031056a820f7c6d287e129bc9fd93/gateway-js/src/index.ts#L382
Shouldn't the health check be done first in `updateComposition()` method? Because they are done after the call to `updateServiceDefinitions()` which obviously fails if a service is down... but shouldn't it be the job of the health check to detect that before?
Indeed the error from:
https://github.com/apollographql/federation/blob/a99f9350ffd031056a820f7c6d287e129bc9fd93/gateway-js/src/index.ts#L399-L403
is never shown since we don't even reach that point (by the way the error message should probably be different on startup).

# Second issue: the error should not be caught
If I am not mistaken, the whole thing is then wrapped in a try/catch which silences the error being thrown.
https://github.com/apollographql/federation/blob/a99f9350ffd031056a820f7c6d287e129bc9fd93/gateway-js/src/index.ts#L536-L540
Indeed we get the error in the logs but no error is actually thrown after so the gateway is still running...

Read gateway-js/src/index.ts around updateComposition(), updateServiceDefinitions(), and the try/catch near lines 536-540. Reproduce with the serviceList containing the unavailable potato service and serviceHealthCheck enabled; done means startup fails and the health-check error is surfaced instead of leaving the gateway running without a valid schema.

Written by the indexing model from the issue text.